1. Requires separated ranking-like database = another server = more $$$ to spend
2. Requires rebuilding score calculation layer so it doesn't affect global ranking = more time wasted
No, there are priorities like osu!lazer. You can create ranking for loved yourself as fan-made project. "I don't know how to code" is not an excuse, it is great opportunity to learn it and realize how much time you need to spend on such small thing. API is available for everyone and it gives access to scores made in loved maps. Idea is not worth spending additional money which can be spent on more important issues like fixing performance point broken algorithms, new client, fixing bugs, making osu! a better place.